1. Field of the Invention
The present invention relates to a lithium ion capacitor.
2. Description of Related Art
A lithium ion capacitor is what is called a hybrid capacitor. In the capacitor, typically, an electrode that physically adsorbs and desorbs ions by holding an active material such as activated carbon onto a collector foils is used as a positive electrode and an electrode that adsorbs and desorbs lithium ions by oxidation-reduction reaction is used as a negative electrode. The development of various applications is expected for the lithium ion capacitor as an electric power storage device that can satisfy both large capacity and high output and, for example, automotive applications of the capacitor have been studied.
In association with the above, Japanese Patent Application Publication No. 2014-160638 (JP 2014-160638 A) describes a binder for electrodes for binding an active material to the collector foil. Although this invention focuses on a lithium-ion battery rather than the lithium ion capacitor, it is developed in consideration of using the battery including a capacitor for automotive applications. A styrene-butadiene rubber (SBR) conventionally used as a binder for electrodes is an aqueous binder and the binder is desirable because the environmental load in the manufacturing process of the electrodes can be easily reduced. In contrast, JP 2014-160638 A points out the problem of oxidative deterioration of SBR when it is used for the positive electrode of a lithium ion battery, and states that an aqueous binder containing a specific constitution unit derived from a monomer having a hydroxy group and a specific constitution unit derived from a polyfunctional (meth)acrylate monomer and having a highly cross-linked structure does not cause oxidative deterioration and performance of this aqueous binder is not affected even under a use environment at a relatively high temperature of 60° C.
For the automotive applications of the lithium ion capacitor, however, a use environment at a higher temperature is assumed. In the case of installation in a vehicle compartment, a lithium ion capacitor having a durability at 85° C. is generally required. In contrast, the binder for electrodes described in JP 2014-160638 A is assumed to be used in a high temperature environment at 60° C. and thus a durability of this binder for electrodes at 85° C. is not secured when the binder is applied to the lithium ion capacitor. Examples of aqueous binders used for electrodes include PEO*PEG (a mixture of polyethylene oxide and polyethylene glycol) and an acrylic-based thermoplastic elastomer. The binding force of these aqueous binders, however, decreases under the high temperature environment at 85° C. When these aqueous binders are used as the binders for the positive electrode of the lithium ion capacitor, significant increase in the resistance generated from the positive electrode has been demonstrated.
Under the high temperature environment at 85° C., an electrolytic solution is also affected by heat. JP 2014-160638 A, however, only focuses on the binder for electrodes and does not consider the durability of the electrolytic solution. Thus, JP 2014-160638 A states that a lithium salt compound, which is commonly used for a lithium ion battery, can be used as an electrolyte. In the specific example, lithium hexafluorophosphate is used. However, it has been found that under the high temperature environment at 85° C. the lithium ion capacitor using lithium fluorophosphate as the electrolyte has significant decrease in capacity and large increase in internal resistance and thus cannot be used.